>Usually that message means that authentication is not turned on or set correctly for the outgoing server. Verify advanced settings, outgoing server, a "Outgoing server requires authentication, use same settings as incoming server" is checked. Assuming since it has been working for so long that an SMTP username and password are set in code and they are correct? If not, then have they suddenly started requiring authentication? (Some versions have "Login using secure password authentication" set that)
>
>The username and password are not set in code. If Outlook Express is configured correctly to send e-mail, CDO will (or at least it always has in the past) use that configuration to send e-mail. As I said, I can successfully send e-mail using OE on their server but I cannot send using CDO.
>
>http://www.answersthatwork.com/Download_Area/ATW_Library/Networking/Network__3-SMTP_Server_Status_Codes_and_SMTP_Error_Codes.pdf
>
>I have alreasy seen this document and it did not help solve the problem.
>
>FWIW, I can successfully send e-mail vis the program on my local machine.

have they installed any new spam filter, firewall, or antivirus app? If you can, capture the running processes on a workstation and the servers.